1. Technical Field
The present invention relates to a printing apparatus, such as a printer of a facsimile machine, a copying machine, and OA equipment, a printing program, a printing method, an image processing apparatus, an image processing program, an image processing method, a recording medium having the program recorded thereon, etc., and particularly, the invention is suitable for a so-called ink-jet type printing apparatus which discharges fine particles of liquid inks of a plurality or colors onto a print sheet (printing medium) to draw predetermined characters and predetermined images, a printing program, a printing method, an image processing apparatus, an image processing program, an image processing method, and a recording medium having the program recorded thereon.
2. Related Art
Hereinafter, a printing apparatus, particularly a printer (hereinafter referred to as “ink jet printer”) employing an ink jet system will be described.
Since ink jet printers generally provide low-price and high-quality color printed materials to be easily obtained, they have been widely used not only in offices but also by general users along with the widespread use of personal computers, digital cameras, and the like.
An ink jet printer is generally adapted such that predetermined characters and predetermined images are printed on a printing medium and thereby a desired printed material is produced by discharging (ejecting) particles of liquid ink in the form of dots from nozzles of a print head while a movable body called a carriage integrally provided with a cartridge and the print head reciprocates in a direction perpendicular to a sheet feed direction of the printing medium (sheet). Also, since the cartridge is provided with ink cartridges for four colors (black, yellow, magenta, and cyan) and a print head for each color, not only monochrome printing but also full color printing by combining the respective colors can be easily performed (furthermore, full color printing by combining six colors, seven colors, or eight colors in which light cyan, light magenta, etc. are added to the above four colors is also put to practical use).
Further, in the ink jet printer of the type that is adapted to perform printing while the print head on the carriage is caused to reciprocate in a direction perpendicular to the sheet feed direction, it is necessary to cause the print head to reciprocate from tens of times to 100 times or more in order to finely print an entire sheet. Therefore, an ink jet printer has a drawback in that the printing time is longer than printing apparatuses using other methods, for example, a laser printer using electrophotography, such as a copying machine.
In contrast, in an ink jet printer of the type in which a long print head having the same (or greater than) dimension as the width of a print sheet, and a carriage is not used, it is not necessary to move the print head in the width direction of the print medium and printing by a so-called one scanning (one pass) is provided. Therefore, the same high-speed printing as a laser printer is provided. Further, since a drive system, etc. for moving the carriage which carries the print head becomes unnecessary, this type of ink jet printer has an advantage in that a printer housing can be made compact and lightweight, and noise reduction also improves significantly. The ink jet printer of the former type is generally called a “multipass type” or a “serial” printer, and the ink jet printer of the latter type is generally called a “line-head type printer” ”
Since the print head of such an ink jet printer is configured such that one row of or plurality of minute nozzles having a diameter of about 10 to 70 μm are arranged in a printing direction at regular intervals, a so-called “flight deflection phenomena” may occur, including a phenomenon that the ink discharge direction of some of the nozzles may be inclined due to manufacturing errors, a phenomenon that some nozzles may be disposed in positions which deviate from ideal positions, and a phenomenon that the landing position of a dot to be formed by the nozzles may deviate from a target point.
As a result, poor printing called a “banding (streak) phenomenon” may be caused in portions where printing is performed using the defective nozzles, thereby noticeably deteriorating printing quality. That is, if the “flight deflection phenomenon” is caused, the distance between dots discharged by adjacent nozzles will become uneven. As a result, a “white streak” (in a case in which a print sheet is white) will be generated in a portion where the distance between adjacent dots is large, and a “dark streak” will be generated in a portion where the distance between adjacent dots is short.
In particular, such a banding phenomenon tends to be caused more noticeably in the “line-head type printer” in which a print head or a printing medium is fixed one pass printing) than in the “multipass type printer” (serial printer) as mentioned above (in the multipass type printer, there is a technique that makes the banding inconspicuous by making the print head reciprocate several times).
Therefore, although research and development in hardware, including improvements in the manufacturing of a print head, improvements in the design thereof, etc. have been made in order to prevent poor printing caused by such a “banding phenomenon,” it is difficult to provide a print head in which the “banding phenomenon” is 100% eliminated from the viewpoint of manufacturing cost, technology, etc.
Thus, under the present circumstances, in addition to improvements in hardware as mentioned above, a technique to reduce such a “banding phenomenon” using a so-called software technique, such as printing control, is also used.
For example, JP-A-6-340094 entitled “Ink-jet Recording Apparatus and Ink-jet Recording Method” suggests a method to reduce a “white streak” extending in a direction perpendicular to a nozzle arrangement direction, which is caused by the “banding phenomenon,” by greatly changing the size of dots in the a driving direction (perpendicular to the nozzle arrangement direction) of a print head while the size of dots in the nozzle arrangement direction of the print head are made the same.
On the other hand, although such a banding phenomenon is caused by the flight deflection phenomenon as mentioned above, it is known that the banding phenomenon is also caused by the so-called “density unevenness.”
That is, this “density unevenness” is a phenomenon in which a predetermined size of dot is no longer printed because the printing position is as normal, but the ink of each nozzle of the print head is not discharged as normal, unlike the printing deviation caused by the flight deflection phenomenon as mentioned above. Accordingly, in a case in which ink is not discharged at all (non-discharge) and the ink discharge amount is less than a specified value, no dot is printed or dots having a smaller size than a prescribed size are printed. Thereby, the density of only a line to be printed by the nozzle becomes low, and consequently a “white streak” or density unevenness similar to the “white streak” is generated in the portion. In contrast, if the ink discharge amount is excessively more than a specified value, dots having a larger size than a prescribed size are printed, and the density of a line to be printed by the nozzle becomes high, and consequently a “dark streak” or density unevenness similar to the “dark streak” is generated in the portion.
Therefore, for example, in JP-A-1-129667 and JP-A-3-162977, the variation with respect to the density data for every nozzle of a print head is considered density variation so as be corrected. Specifically, JP-A-1-129667 and JP-A-3-162977 suggest a method of trying to reduce any density variation by correcting the pixel value (density value) of a pixel of the image data corresponding to a nozzle whose ink discharge amount is less than a specified value so that the pixel value may be increased to a value greater than its original value, and in contrast, by correcting the pixel value (density value) of a pixel of the image data corresponding to a nozzle whose ink discharge amount is more than a specified value so that the pixel value may be decreased to a value smaller than its original value.
Further, for example, JP-A-2002-19101, JP-A-2003-136702, and JP-A-2003-63043 suggest a method of trying to reduce the density unevenness of a portion with dark density by using other colors for the color of the portion or increasing the ink discharge amount of a nozzle adjacent to a non-discharge nozzle with respect to a discharge failure phenomenon of a nozzle which does not discharge ink at all.
Meanwhile, in the above related art, with respect to the banding phenomenon resulting from either the “flight deflection” or the “density unevenness” caused by poor discharge, the effect of reducing the banding phenomenon can be exhibited somewhat, but it cannot be said that the effect is satisfactory. Further, it turned out that it is difficult to satisfactorily cope with the banding phenomenon resulting from both phenomena.
For example, although it is possible to reduce a “white streak” extending in a direction perpendicular to a nozzle arrangement direction by greatly changing the size of a dot as mentioned above with respect to the banding phenomenon resulting from “flight deflection,” there is a problem in that the density of the portion becomes high and “density unevenness” is caused.
Meanwhile, although the method of using other colors for the color of a portion with dark density as mentioned above with respect to the banding phenomenon resulting from the “density unevenness” caused by poor discharge, can reduce a color difference, it cannot be said that the method is suitable for printing which requires high image quality because a color tone may change. Further, although the method of increasing the ink discharge amount of a nozzle adjacent to a non-discharge nozzle is effective in a portion with thin density, the ink discharge amount of the adjacent nozzle cannot be increased any more in a portion with dark density, but banding will still remain.